1. Field of the Invention
The present invention relates to a toner composition for development of an electrostatic image in the electrophotographic process, electrostatic recording process, electrostatic printing process and the like.
2. Discussion of Related Art
In development of an electrostatic latent image in electrophotography, toner particle size and toner particle size distribution are known to serve as important factors to obtain high resolution and high image quality.
When the particle size of a toner is reduced, the amount of pulverizing energy required increases as the size decreases, which generally leads to reduction in productivity and rise in cost; it is therefore necessary to use a resin with excellent fixing property and good pulverizability as the main component of the binder resin component of the toner.
However, it has been pointed out that even when the size of the toner is reduced while improving the pulverizing capability of the resin itself, the fluidity is reduced due to an increase in friction and aggregation of the toner particles and an increase in the ratio of water adhering to the surface of the toner particles under high humidity conditions, which results in a problem of reduction in developability accompanying reduction in the chargeability and transportability of the toner, because the surface area per unit weight of the toner increases.
Another problem has been pointed out that even when the particle size is reduced, the ratio of very fine particles having a particle diameter of not more than 5 .mu.m increases and the particle size distribution broadens so that the uniformity in the electric charge of the toner is lowered.
To solve these problems, Japanese Patent Laid-Open Nos. 72054/1979 and 129437/1983 propose toners wherein the particle size distribution is controlled to reduce the number % of particles having a diameter of not more than 5 .mu.m to suppress reduction in the fluidity and improve fluctuation in the amount of electric charge of the toner.
However, no satisfactory effect is obtained simply by reducing the number % of particles having a diameter of not more than 5 .mu.m; fluidity and chargeability remain to be further improved.
Japanese Patent Laid-Open No. 284151/1990 proposes a toner containing toner particles having an average particle size of from 4 to 6 .mu.m, being not less than 60 number % of toner particles which have a diameter of not more than 5 .mu.m, and a fine powder of an inorganic compound. Although such a toner makes it possible to obtain a sharp image, it is reported that the amount of a fine powder of the inorganic compound added must be increased because the number % of toner particles having a diameter of not more than 5 .mu.m is high.
Although various types of such fine powder of an inorganic compound are traditionally known, fine powder of silicone dioxide (silica) has been generally used to add and mix with the toner powder, as a surface treating agent.
However, because fine powder of silica is hydrophilic when it is directly used, it absorbs moisture from the air under high temperature and high humidity conditions, and this decreases the fluidity or causes aggregation of the toner particles. For this reason, it has been proposed to use silica fine powder treated by a hydrophobic treatment (See Japanese Patent Laid-Open Nos. 5782/1971 and 47345/1973). For example, a dimethyl substitution product has been known, in which a methyl group of a silane is bonded with silica by a reaction of dimethyldichlorosilane with hydrophilic silica (R-972: Nippon Aerosil Co., Ltd.).
However, the fine powder of silica is not hydrophobic enough even it has been treated to have a hydrophobic property. The aggregation property is noted at high temperature and high humidity and the fluidity of the toner is decreased. Thus, the degree of hydrophobic property has become an important issue.
Specifically, in the case of R-972, for example, the silanol group of the hydrophilic silica is substituted 70 to 80%, and the remaining 20 to 30% of silanol groups are not substituted and remain unchanged, and the degree of hydrophobicity is only 40.
Therefore, it has been pointed out that, when silica fine powder having such a degree of hydrophic property is used with the toner composition, it is difficult to stably form a great number of visible images with good quality for a long period by such a toner.
More recently, there have been several proposals to solve these problems. In one case the stable formation of a visible image with good quality in forming a great number of visible images for a long period can be obtained when hydrophobic silica fine powder having a hydrophobic index (i.e. a degree of hydrophobic property) of not less than 50, or more preferably not less than 65, which is obtained through a hydrophobic treatment of organic silicon compounds having a specific organic group, is added and mixed with toner powder in an amount of 0.01 to 15% by weight (Japanese Patent Laid-Open No. 81650/1984). A second proposal is to provide a toner containing 0.01 to 20% by weight of a hydrophobic silica fine powder obtained through a hydrophobic treatment, so that the degree of hydrophobicity is within the range of 30 to 80 (Japanese Patent Laid-Open No. 231552/1984).
Such a hydrophobic treatment has been used in methods already known, in which a chemical treatment is performed by an organic silicon compound reacting or physically adsorbing silica fine powder. In general, a method is adopted by which a treatment is performed by an organic silicon compound at the same time when or after silica fine powder obtained by a vapor phase oxidation of a silicon halogen compound has been treated by a silane coupling agent.
However, hydrophobic silica heretofore considered to show a high hydrophobic property has a hydrophobic degree of less than 80 at most, and actually those described in the above patent publication (Japanese Patent Laid-Open No. 231552/1984) has a hydrophobic degree of up to 74.
Japanese Patent Laid-Open No. 81650/1984 describes a compound with a degree of hydrophobic property of more than 65 as a high hydrophobic compound, whereas the upper limit is not clear, and it is not known exactly how high the hydrophobic property of the compound disclosed in the above patent publication is. The hydrophobic silica having a hydrophobic degree of less than 80, at best shows the improvements in electric charge retainability and fluidity compared with the conventional dimethyl substituted product having a hydrophobic degree of from 40 to 42. This was not sufficient for the purpose, however, under high temperature and high humidity conditions, because electric charge retainability and fluidity decreased and the stable formation of a visible image with good quality was hindered.
In the chase when the degree of hydrophobic property is not enough, a number of unreacted silanol groups remain in the hydrophobic silica or, in the case when the substitutents reacted with the silanol groups are small groups of atoms as a whole, a stable hydrogen bond is formed between the carboxyl group in the binder resin of the toner particles and moisture in the surroundings with the other unreacted silanol groups. As a result, the above stated problems arise under high temperature and high humidity conditions.
Therefore, whether the degree of hydrophobic property is high enough is determined by which kind of hydrophilic groups the binder resin has.
As the binder resin for toner, in general various types of resins are used including styrene type polymers such as polystyrene, styrene-butadiene copolymer, styrene-acrylic copolymer, etc., ethylene type polymers such as polyethylene, ethylene-vinyl acetate copolymer, etc., poly-(meth)acrylic acid esters, polyester resins, epoxy resins, and polyamide resin, etc. Of these resins for those having naturally high hydrophobic properties, such as normal styrene-acryl resin, a high degree of hydrophobic property will not be required of the silica. In the case of the polyester resin obtained by condensation polymerization of alcohol and carboxylic acid, because many carboxyl groups, which are hydrophilic groups, are contained in this resin, hydrogen bonds of such groups with water causes the decrease of electric charge retainability and fluidity of the toner. Thus, it has been pointed out that the degree of hydrophobic property is not sufficient.
Above all, when using a polyester resin as the major component of the binder resin of the toner and the toner size is reduced to obtain high resolution and high image quality, as described above, the surface area per unit weight of the toner increases, and the toner becomes more susceptible to the effect of moisture in the environment, which results in reduction in fluidity. For this reason, it is necessary to add a surface treating agent, such as hydrophobic silica fine powder, to obtain sufficient fluidity.
In such case, it is necessary to add a larger quantity of hydrophobic silica to maintain the fluidity of toner particles in the conventional type hydrophobic silica. For example, in the above patent publication (Japanese Patent Laid-Open No. 81650/1984), which describes the compound classified as a high hydrophobic compound group, with a hydrophobic index of 50 or more, it is proposed to add hydrophobic silica in an amount of 0.01 to 15% by weight. In the above patent publication (Japanese Patent Laid-Open No. 231552/1984) describing a compound with a hydrophobic index of 30 to 80, it is proposed to add hydrophobic silica in an amount of 0.01 to 20% by weight.
However, there remains the problems that, if the amount of hydrophobic silica is increased, the isolated silica causes damage to the surface of the photoconductor drum and the silica causes black spots as the initiator, even if the fluidity is maintained. The black spot is a type of filming on a photoconductor drum and it appears as black points on a visible image. Because the particles of hydrophobic silica are considerably hard, this phenomenon remarkably appears when a photoconductor drum used is a substance of relatively low hardness, such as a selenium-tellurium type or an organic photoconductor drum. Further, the same problem occurs even in the case of a selenium-arsenic type substance, which is relatively hard but is brittle to mechanical shock.
Another problem has been pointed out that when the additional amount of hydrophobic silica is great, the fluidity of toner tends to decrease because the moisture resistance of the hydrophobic silica is insufficient when used under high temperature and high humidity conditions.
Accordingly, it is preferred that the additional amount of hydrophobic silica be as low as possible, and it is also preferred to use such hydrophobic silica, which can improve electric charge retainability and fluidity of the toner by adding it in very small quantities.
On the other hand; a hydrophobic treatment of silica has been performed in the past through the use of volatile silanes in a reactor heated at about 400.degree. C. For example, a method to utilized the thermal decomposition oxidizing reaction in a oxyhydrogen flame of silicon tetrachloride gas has been used, wherein the following reaction occurs: EQU SiCl.sub.4 +2 H.sub.2 +O.sub.2 .fwdarw.SiO.sub.2 +4 HCl
In the meantime, because it is not very easy to remove hydrogen chloride generated during this reaction, it has been pointed out that the pH value of the hydrophobic silica thus obtained decreases to about 3 to 4, and problems such as the rusting on the inner wall of the tank for the hydrophobic silica-toner facilities during long-term use arises.
Specifically, the conventional hydrophobic silica obtained in the past had various problems such as the suitability of the degree of hydrophobic property and the amount to be added and, in addition to these problems, counter measures are urgently needed to improve the acidification condition of hydrophobic silica fine powder caused by a hydrogen chloride generated during treatment.